Piezoelectric ceramic composition and piezoelectric ceramic device composed of same

ABSTRACT

A piezoelectric ceramic composition that is based on a layered bismuth compound composed of Sr, Bi, Nb, oxygen, contains an additional monovalent metallic element. The piezoelectric ceramic composition has an elevated Curie point, is highly reliable at higher temperatures, that is, minimizes the reduction in the piezoelectric effect, and is useful as a material for piezoelectric ceramic devices that contain little or no lead or lead compounds. The layered bismuth compound contains not more than about 0.125 mol and more than 0 mol of the monovalent metallic element for 1 mol of Nb.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a piezoelectric ceramic composition anda piezoelectric ceramic device composed of the piezoelectric ceramiccomposition. In particular, the present invention relates to apiezoelectric ceramic composition that is useful as a material forpiezoelectric ceramic devices, such as a piezoelectric ceramic filter, apiezoelectric ceramic resonator and a piezoelectric ceramic oscillator,and to a piezoelectric ceramic device composed of the piezoelectricceramic composition.

2. Description of the Related Art

Hitherto, a lead zirconate titanate, (Pb(Ti_(x)Zr_(1-x))O₃) or a leadtitanate- (PbTiO₃) based piezoelectric ceramic composition has beenwidely used in piezoelectric ceramic devices, such as a piezoelectricceramic filter, a piezoelectric ceramic resonator and a piezoelectricceramic oscillator. However, the lead zirconate titanate- or the leadtitanate-based piezoelectric ceramic composition contains a large amountof lead, which vaporizes as lead oxide during production of thepiezoelectric ceramic device, and thereby results in poor productuniformity. Thus, a piezoelectric ceramic composition that containslittle or no lead is desired to overcome this problem. In addition, alower amount of lead is also desirable in view of environmentalpollution.

On the other hand, a piezoelectric ceramic composition based on alayered bismuth compound, such as SrBi₂Nb₂O₉, is free of lead oxide anddoes not cause such problems.

In addition, SrBi₂Nb₂O₉-based materials, as disclosed in JapaneseUnexamined Patent Application Publication No. 2001-328865, exhibit asignificantly small change in frequency when temperature changes, andtherefore have received attention as piezoelectric materials forresonators recently.

While the piezoelectric ceramic device is typically used at atemperature range, for example, from 60° C. to 200° C., those that canbe used at a higher temperature of, for example, about 400° C., aredesired for use in a resonator. Since the piezoelectric ceramicresonator cannot be used above its Curie point, where it has nopiezoelectric effect, the piezoelectric ceramic resonator must has aCurie point higher than operating temperatures.

According to M. J. Forbess et al. (Applied Physics Letters, Vol. 76,2943, (2000)), SrBi₂Nb₂O₉ has a Curie point of 418° C. and has a loweredpiezoelectric effect when used for a piezoelectric ceramic resonator ata temperature close to 400° C. A preferred Curie point in this case isat least 430° C.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide apiezoelectric ceramic composition that is based on a layered bismuthcompound composed of Sr, Bi, Nb, oxygen, and an additional monovalentmetallic element. The piezoelectric ceramic composition has an elevatedCurie point, is highly reliable at higher temperatures, that is,minimizes the reduction in the piezoelectric effect, and is useful as amaterial for piezoelectric ceramic devices that contain little or nolead or lead compounds.

The piezoelectric ceramic composition according to the present inventioncontains not more than about 0.125 mol (and more than 0 mol) ofmonovalent metallic element per 1 mol of Nb. This amount of monovalentmetallic element leads to an increased Curie point of the piezoelectricceramic composition. However, more than about 0.125 mol of monovalentmetallic element will adversely decrease the Curie point of thepiezoelectric ceramic composition.

Preferably, the monovalent metallic element used in the presentinvention is at least one selected from the group consisting of Li, Naand K, which give further advantages of the present invention.

Furthermore, the piezoelectric ceramic composition according to thepresent invention preferably contains not more than about 0.175 mol (andmore than 0 mol) of trivalent metallic element other than Bi per 1 molof Nb. When the ceramic device composed of the piezoelectric ceramiccomposition is used as a resonator, this amount of trivalent metallicelement other than Bi gives a practicable Q_(max) factor (the maximumelectrical quality factor Q (1/tan δ) within a band, that is, atfrequencies between the resonance frequency and the anti-resonancefrequency).

Preferably, the trivalent metallic element other than Bi is at least oneselected from the group consisting of Sc, Y, La, Ce, Nd, Sm, Gd, Dy, Erand Yb, which give further advantages of the present invention. Morepreferably, the trivalent metallic element other than Bi is Nd, whichgive still further advantages of the present invention.

In the main component of the piezoelectric ceramic composition accordingto the present invention, not more than about 10 molar percent (and morethan 0 molar percent) of Nb may be replaced with Ta. As a result, theamount of Ta will be up to 10% based on the total mols of Nb and Tapresent. Replacing more than about 10 molar percent of Nb with Ta willresult in a too low Q_(max) factor for the piezoelectric ceramiccomposition to function as a resonator.

Further, the main component of the piezoelectric ceramic compositionaccording to the present invention may contain not more than about 0.01mol (and more than 0 mol) of Mn per 1 mol of the main component.Manganese in amounts larger than that will result in a too low Q_(max)factor for the piezoelectric ceramic composition to function as aresonator.

The piezoelectric ceramic device according to the present inventionincludes a piezoelectric ceramic that is composed of piezoelectricceramic composition according to the present invention, and electrodeson the piezoelectric ceramic.

The foregoing and other objects, features, and advantages of the presentinvention will be more apparent from the detailed description of thefollowing embodiment according to the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a piezoelectric ceramic oscillatoraccording to an embodiment of the present invention; and

FIG. 2 is a sectional view of the piezoelectric ceramic oscillator shownin FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

SrCO₃, Bi₂O₃, Nb₂O₅, Ta₂O₅, Na₂CO₃, K₂CO₃, Li₂CO₃,Nd₂O₃, La₂O₃,Ce₂O₃,Sc₂O₃, Y₂O₃, Sm₂O₃, Gd₂O₃, Dy₂O₃, Er₂O₃, Yb₂O₃ and MnCO₃ were firstlyprepared as starting materials. These compounds were weighed to meet thecomposition formula (Sr_(a)Bi_(b)Nb_(c)O₉+w mol M1+x mol M3+y mol Ta+zmol MnCO₃ (wherein, M1 is Na, K or Li, M3 is Nd, La, Ce, Sc, Y, Sm, Gd,Dy, Er or Yb, and a, b, c, w, x, y and z are as shown in Tables 1 and2)) and were wet-blended in a ball mill for about 16 hours. Theresulting mixture was dried, and was then calcined at 800 to 1000° C.The product was mixed with an organic binder, a dispersant, ananti-foaming agent, a surfactant and pure water in proper quantities,and was pulverized in the ball mill. The resulting slurry was appliedwith a doctor blade into sheets 40 to 80 μm in thickness. Electrodeswere printed on some of these sheets with a Pt paste, and then theprinted sheets were dried. The printed sheets and other sheets werestacked. The resulting laminate was compacted and was baked at 1100 to1300° C. Then, the laminate was polarized in an insulating oil at 100 to200° C. under 5 to 10 kV/mm dc voltage for 10 to 30 min, yielding anenergy-confinement piezoelectric ceramic oscillator 10 (sample) shown inFIGS. 1 and 2.

A piezoelectric ceramic oscillator 10 shown in FIGS. 1 and 2 includes apiezoelectric ceramic 12 in, for example, a rectangular parallelepipedshape. The piezoelectric ceramic 12 is polarized in the direction fromthe bottom face to the top face as indicated by an arrow. Thepiezoelectric ceramic 12 has vibrating electrodes 14 a and 14 b on itstop and bottom faces, respectively. The vibrating electrodes 14 a and 14b are of, for example, a circular shape and are disposed at the centerof each face. Thus, the vibrating electrode 14 b is disposed right belowthe vibrating electrode 14 a. The piezoelectric ceramic 12 also has aninternal vibrating electrode 14 c in, for example, a circular shape. Thevibrating electrode 14 c is disposed in the middle of the vibratingelectrodes 14 a and 14 b. Thus, the vibrating electrodes 14 a, 14 b and14 c are vertically aligned. Leading electrodes 16 a, 16 b and 16 c in,for example, a T shape are disposed between their respective vibratingelectrodes 14 a, 14 b and 14 c, and a side face of the piezoelectricceramic 12. Specifically, the leading electrodes 16 a and 16 b aredisposed between their respective vibrating electrodes 14 a and 14 b,and one side face of the piezoelectric ceramic 12, and the leadingelectrode 16 c is disposed between the vibrating electrode 14 c and theother side face of the piezoelectric ceramic 12. A voltage is appliedbetween the leading electrodes 16 a and 16 b and the leading electrode16 c to cause electric potential difference between the exteriorvibrating electrodes 14 a and 14 b and the interior vibrating electrode14 c, and thus to excite a thickness-longitudinal vibration secondharmonic mode.

The piezoelectric ceramic oscillator 10 (sample) was tested for theQ_(max) factor at room temperature in the thickness-longitudinalvibration second harmonic mode. In addition, the temperature dependenceof permittivity and the Curie point were measured. The results are shownin Tables 1 and 2.

TABLE 1 Samples with asterisks are outside of the scope of the presentinvention. Sample Curie No. a b c M1 w M3 x y z w/c x/c point (° C.)Q_(max)  1* 1.0 2.0 2.0 — 0 — 0 0 0 0 0 418 9.8  2 1.0 2.0 2.0 Na 0.05 —0 0 0 0.025 0 480 10.1  3 1.0 2.0 2.0 Na 0.1 — 0 0 0 0.05 0 480 10.5  41.0 2.0 2.0 Na 0.2 — 0 0 0 0.1 0 455 10.4  5 1.0 2.0 2.0 Na 0.25 — 0 0 00.125 0 435 10.3  6* 1.0 2.0 2.0 Na 0.3 — 0 0 0 0.15 0 360 9.6  7 1.02.0 2.0 K 0.1 — 0 0 0 0.05 0 480 10.2  8 1.0 2.0 2.0 K 0.2 — 0 0 0 0.1 0450 10.5  9* 1.0 2.0 2.0 K 0.3 — 0 0 0 0.15 0 370 8.9 10 1.0 2.0 2.0 Li0.1 — 0 0 0 0.05 0 480 9.8 11 1.0 2.0 2.0 Li 0.2 — 0 0 0 0.1 0 435 10.3 12* 1.0 2.0 2.0 Li 0.3 — 0 0 0 0.15 0 370 8.6 13 1.0 2.0 2.0 Na 0.05 Nd0.05 0 0 0.025 0.025 490 12.5 14 1.0 2.0 2.0 Na 0.1 Nd 0.2 0 0 0.05 0.1470 11.5 15 1.0 2.0 2.0 Na 0.1 Nd 0.35 0 0 0.05 0.175 450 10.3 16 1.02.0 2.0 Na 0.1 Nd 0.4 0 0 0.05 0.2 435 8.9 17 0.9 2.0 2.0 Na 0.1 Nd 0.10 0 0.05 0.05 470 15.2 18 0.8 2.0 2.0 Na 0.1 Nd 0.2 0 0 0.05 0.1 46512.3 19 0.8 2.0 2.0 Na 0.15 Nd 0.3 0 0 0.075 0.15 460 11.4 20 0.8 2.22.0 Na 0.15 Nd 0.3 0 0 0.075 0.15 470 13.3 21 0.9 2.0 2.0 Na 0.1 La 0.10 0 0.05 0.05 460 14.6

TABLE 2 Sample Curie No. a b c M1 w M3 x y z w/c x/c point (° C.)Q_(max) 22 0.8 2.0 2.0 Na 0.1 La 0.2 0 0 0.05 0.1 440 13.2 23 0.9 2.02.0 Na 0.1 Sc 0.1 0 0 0.05 0.05 470 14.3 24 0.8 2.0 2.0 Na 0.1 Sc 0.2 00 0.05 0.1 460 13.2 25 0.9 2.0 2.0 Na 0.1 Y 0.1 0 0 0.05 0.05 475 15.526 0.8 2.0 2.0 Na 0.1 Y 0.2 0 0 0.05 0.1 460 14.1 27 0.9 2.0 2.0 Na 0.1Sm 0.1 0 0 0.05 0.05 470 13.5 28 0.8 2.0 2.0 Na 0.1 Sm 0.2 0 0 0.05 0.1455 12.8 29 0.9 2.0 2.0 Na 0.1 Dy 0.1 0 0 0.05 0.05 470 14.3 30 0.8 2.02.0 Na 0.1 Dy 0.2 0 0 0.05 0.1 460 13.7 31 0.9 2.0 2.0 Na 0.1 Yb 0.1 0 00.05 0.05 475 13.9 32 0.8 2.0 2.0 Na 0.1 Yb 0.2 0 0 0.05 0.1 465 13.1 330.9 2.0 2.0 Na 0.1 Ce 0.1 0 0 0.05 0.05 480 13.5 34 0.8 2.0 2.0 Na 0.1Ce 0.2 0 0 0.05 0.1 460 13.0 35 0.9 2.0 2.0 Na 0.1 Gd 0.1 0 0 0.05 0.05485 13.9 36 0.8 2.0 2.0 Na 0.1 Gd 0.2 0 0 0.05 0.1 470 12.9 37 0.9 2.02.0 Na 0.1 Er 0.1 0 0 0.05 0.05 475 13.4 38 0.8 2.0 2.0 Na 0.1 Er 0.2 00 0.05 0.1 470 13.0 39 1.0 2.0 1.9 Na 0.05 Nd 0.05 0.1 0 0.0263 0.0263460 13.1 40 1.0 2.0 1.8 Na 0.05 Nd 0.05 0.2 0 0.0277 0.0277 440 12.6 411.0 2.0 2.0 Na 0.05 Nd 0.05 0 0.005 0.025 0.025 490 13.8 42 1.0 2.0 2.0Na 0.05 Nd 0.05 0 0.01 0.025 0.025 480 14.1

The Q_(max) factor was determined for each sample under the conditions(calcination temperature, firing temperature, temperature of insulatingoil during polarization, and dc voltage) that exhibited the largestQ_(max) factor. The Q_(max) factor depended on the shape of the sample,the mode of vibration and the type of the electrode. The applications ofthe piezoelectric ceramic device, in particular, a piezoelectric ceramicresonator, at a higher temperature according to the present inventionare of very special use, and a high Q_(max) factor as required ingeneral-purpose piezoelectric ceramic devices, in particular,piezoelectric ceramic resonators that are used in household electricappliances, is not required. This is because even a low Q_(max) factoris practicable depending on the circuit design. Under the presentconditions, a Q_(max) factor of at least 10 at room temperature is apractical level.

The Curie points in Tables 1 and 2 were determined for each sample underthe conditions (calcination temperature and firing temperature) thatgave the highest density. When the piezoelectric ceramic device is usedat a high temperature close to 400° C., a Curie point of at least 430°C. is required for practical use.

It is apparent from Tables 1 and 2 that the piezoelectric ceramiccompositions within the scope of the present invention have Curie pointshigher than 430° C. and thus are useful materials for piezoelectricceramic devices, in particular, piezoelectric ceramic resonators at ahigh temperature close to 400° C.

The samples that are within the scope of the present invention andcontain not more than about 0.175 mol (and more than 0 mol) of trivalentmetallic elements other than Bi per 1 mol of Nb have Q_(max) factors ofnot less than the practical level of 10, and thus are useful materialsparticularly for piezoelectric ceramic resonators.

The piezoelectric ceramic composition according to the present inventionis not limited to the embodiment described above, and is effectivewithin the scope of the present invention.

The present invention can be applied not only to the piezoelectricceramic oscillator 10 described above, but also to other piezoelectricceramic devices, such as piezoelectric ceramic oscillators,piezoelectric ceramic filters and piezoelectric ceramic resonators.

1. A piezoelectric ceramic composition comprising a layered bismuthcompound comprising Sr, Bi, Nb and oxygen, and a positive amount ofmonovalent metallic element, wherein the amount of monovalent metallicelement is not more than about 0.125 mol for each mol of Nb.
 2. Thepiezoelectric ceramic composition according to claim 1, wherein themonovalent metallic element is at least one member selected from thegroup consisting of Li, Na and K.
 3. The piezoelectric ceramiccomposition according to claim 2, wherein the layered bismuth compoundfurther comprises a positive amount of not more than about 0.175 mol oftrivalent metallic element other than Bi for each mol of Nb.
 4. Thepiezoelectric ceramic composition according to claim 3, wherein thetrivalent metallic element is at least one member selected from thegroup consisting of Sc, Y, La, Ce, Nd, Sm, Gd, Dy, Er and Yb.
 5. Thepiezoelectric ceramic composition according to claim 3, wherein thetrivalent metallic element is Nd.
 6. The piezoelectric ceramiccomposition according to claim 4, further comprising a positive amountTa up to about 10 molar percent based on the moles of Nb and Ta present.7. The piezoelectric ceramic composition according to claim 6, furthercomprising a positive amount Mn up to about 0.01 mol.
 8. A piezoelectricceramic according to claim 7 having a Curie point of at least about 430°C. and a Q_(max) of at least about
 10. 9. A piezoelectric ceramic devicecomprising a piezoelectric ceramic composed of the piezoelectric ceramiccomposition according to claim 7 in combination with electrodes.
 10. Thepiezoelectric ceramic composition according to claim 1, wherein thelayered bismuth compound further comprises a positive amount of not morethan about 0.175 mol of trivalent metallic element other than Bi for 1mol of Nb.
 11. A piezoelectric ceramic device comprising a piezoelectricceramic composed of the piezoelectric ceramic composition according toclaim 10 in combination with electrodes.
 12. The piezoelectric ceramiccomposition according to claim 10, wherein the trivalent metallicelement is at least one member selected from the group consisting of Sc,Y, La, Ce, Nd, Sm, Gd, Dy, Er and Yb.
 13. The piezoelectric ceramiccomposition according to claim 10, wherein the trivalent metallicelement is Nd.
 14. A piezoelectric ceramic device comprising apiezoelectric ceramic composed of the piezoelectric ceramic compositionaccording to claim 13 in combination with electrodes.
 15. Thepiezoelectric ceramic composition according to claim 1, furthercomprising a positive amount Ta up to about 10 molar percent based onthe mols of Nb and Ta present.
 16. A piezoelectric ceramic devicecomprising a piezoelectric ceramic composed of the piezoelectric ceramiccomposition according to claim 15 in combination with electrodes. 17.The piezoelectric ceramic composition according to claim 1, furthercomprising a positive amount Mn up to about 0.01 mol.
 18. Apiezoelectric ceramic device comprising a piezoelectric ceramic composedof the piezoelectric ceramic composition according to claim 17 incombination with electrodes.
 19. A piezoelectric ceramic devicecomprising a piezoelectric ceramic composed of the piezoelectric ceramiccomposition according to claim 1 in combination with electrodes.
 20. Apiezoelectric ceramic according to claim 1 having a Curie point of atleast about 430° C. and a Q_(max) of at least about 10.